Recent Trends in Philosophy of Time

Kevin Harris: Thank you so much for being here for the podcast with Dr. William Lane Craig. This is Reasonable Faith; I'm Kevin Harris. This ought to be right up your ally, Dr. Craig, what we're going to talk about today. You've written extensively on time and eternity, philosophy of time, and Sean Carroll was recently interviewed on his theory of time, asked the question 'what is time?' I understand that Sean Carroll is a well-known physicist.

Dr. Craig: Right. He's at Cal Tech and a very brilliant mind, and so bears listening to when he speaks on these issues.

Kevin Harris: His book is From Eternity to Here: The Quest for the Ultimate Theory of Time. Wow. It's an attempt to bring the theory of time and the universe to physicists and non-physicists alike. Presenting this he sat down with Wired Magazine to explain his theories, and why Marty McFly's adventures could never exist in the real world—where time only moves forward and never back. So he, like you, sees time travel as impossible.

Dr. Craig: Right, though for quite different reasons, but that's true—that is his conclusion.

Kevin Harris: He says that he's trying to understand how time works. It's a huge question, has lots of different aspects to it. Some of these things go back to Einstein, some of his theories. But it gets down to the arrow of time. Now, what do we mean by that?

Dr. Craig: The arrow of time has to do with the structure of time from earlier to later. Time is what philosophers of time or physicists call anisotropic, that is to say it has a direction. Time is not like space. Points in space are related by a betweenness relation. If you have three points on a line, x, y, and z, one of those points is between the other two, but there isn't any inherent directionality to space. You can go in either direction. There isn't any intrinsic ordering in a direction of the points in space. But instants in time are different. Instants in time have an intrinsic ordering relation of earlier than and later than. And this provides an anisotropy to time, or an asymmetry to time, a sort of direction. Now Carroll doesn't always seem to use the word 'arrow of time' consistently in that way, but this is the burden, I think, of his question—he's trying to understand why is time anisotropic, why does time have a directionality to it?

Kevin Harris: So this gets back to Boltzmann who figured out entropy. Entropy is a measurement of how disorderly things are. How does entropy play into this theory of time.

Dr. Craig: This is a typical physicists answer to the question of the arrow of time. And I think philosophically it's inadequate. He says that entropy tends to grow, that's the second law of thermodynamics. In other words entropy goes up with time. As time goes on entropy increases. And so many physicists, and Carroll himself, I think, simply want to identify the direction of time with the direction of entropy increase. But the problem, Kevin, is that that's circular because how do you know that the direction in which entropy increases is the later than direction, rather than the earlier than direction? You have already assumed in saying that entropy increases that there is a direction between earlier and later, and that entropy is increasing in the later than direction. So at best, I think, increase in entropy is a sort of physical manifestation of time's asymmetry, but it isn't definitional for the asymmetry of time. As philosophers of science like Lawrence Sklar have pointed out, to try to define the anisotropy of time as the direction of entropy increase is to reason in a circle, it's to beg the question, because you're already assuming that time is asymmetrical and that there is a direction in which the entropy is going up. So really I think this project, as he stated it, is ultimately unsuccessful. Entropy can at best be evidence of an arrow of time, but it can't be definitive of it. It may show a direction, Kevin; but what I'm saying is it doesn't constitute the direction of time. Time has an intrinsic directionality in it from earlier to later,[1] and it is in the later than direction that entropy increases. But if entropy just defines the direction of time you could have flipped it around and said, well, we'll call the direction in which entropy increases the earlier than direction, and the entropy in which it goes down, we'll call that the later than direction. It would be just completely symmetric. But it's not symmetric, there's a directionality to time from earlier to later, and it is in the later than direction that we observe entropy to increase. So the direction of entropy increase is actually something that presupposes the asymmetry of time already.

Kevin Harris:Wired Magazine asks, “So the Big Bang starts it all, but you theorize there is something before the Big Bang, something that makes it happen. What's that?” Carroll says, “If you find an egg in your refrigerator you're not surprised. You don't say, 'wow, that's a low entropy configuration, that's unusual,' because you know that the egg is not alone in the universe—it came out of a chicken, which is part of a farm, which is part of the biosphere, etc., etc. But with the universe we don't have that appeal to make. We can't say that the universe is part of something else. But that's exactly what I'm saying, and I'm fitting in with a line of thought in modern cosmology that says that the observable universe is not all there is—it's part of a bigger multiverse. The Big Bang was not the beginning. And if that's true it changes the question you're trying to ask. It's not 'why did the universe begin with low entropy?' It's 'why did part of the universe go through a phase with low entropy?'—and that might be easier to answer.”

Dr. Craig: So what he's suggesting is that when we ask the question 'why was the Big Bang characterized by this extraordinarily low entropy condition?' we'll answer that by embedding the universe in a higher reality – or a larger reality – which is the multiverse. And the universe, he says, is just a part of this multiverse, and has emerged from this vacuum situation of empty space and time that is characterized with a certain energy level and that fluctuations occur in this, which spawns baby universes, so to speak. Now, it's not clear exactly which sort of multiverse model he's adopting here, but this is a fairly traditional multiverse sort of proposal to avoid an absolute beginning of the universe.

Kevin Harris: Once again, Bill, this seems to be evidence of multiverses becoming more popular. There was a time I wondered, just from observing it, if it was just going to go away, but here's another physicist who is embracing some kind of a multiverse.

Dr. Craig: Right. It's an attempt to avoid, in this case, an absolute beginning of the universe in which the low entropy is simply put in as an initial condition as part of the creation of the universe. You try to get rid of the initial creation event by embedding the universe in a larger condition. But the question here, Kevin, I think, that needs to be posed is: can this larger multiverse, then, be extrapolated back to infinity? Can this have been eternal? And I think there's reason to think that it cannot. Look what Carroll says about this empty space from which the universe emerges. He says, “If time flows without entropy, and there's no one there to experience it, is there still time? Yes—there's still time. It's still part of the fundamental laws of nature, even in that part of the universe. It's just that events that happen in that empty universe don't have causality. It's just random fluctuations.” So he's admitting this primordial condition is a temporal state, time is passing, events are still ordered as earlier and later than, but he thinks this is an arena of random fluctuations. I think it would be a mistake to say it's utterly acausal, otherwise you'd have to say there's no causal connection between this primordial condition of the universe and our universe, which would make them utterly diverse and unrelated. There's clearly some sort of causal connection here, even if it involves random fluctuations of the energy.

So the question is: can this state – which he admits is a temporal state – be extrapolated back to past infinity? Can it have existed eternally? And I think there's good reason to think not. He admits, as he says here, “almost nothing happens in this state.” But he says, “there's no way for the universe to be truly static once and for all. There is no state the universe could be in that would just stay put forever and ever and ever.”[2] And that's the key point. This quantum vacuum state is not static, it is not stable, it cannot endure for infinite time. There are these random fluctuations going on in it, and at every point in the vacuum there is a positive non-zero probability that a fluctuation will form there, which will turn into a universe and produce a universe. But then by now every point in the quantum vacuum ought to have produced a universe, and we ought to be observing an infinitely old universe, rather than our relatively young one. So this primordial condition isn't stable. It's not capable of being extrapolated to past eternity. It would have an end after a finite amount of time. So as he says in the interview it's not completely stable, it has a half-life, it will decay, so that it can't exist for infinite time. So as James Sinclair points out in an article that we recently co-authored in response to Brian Pitts on non-singular spacetimes and the origin of the universe, these types of model that postulate an eternal static state prior to the Big Bang ultimately will not work because they can't be extrapolated to past infinity. That state is unstable, it will decay in a finite amount of time, and will therefore end. And that, I think, presents a real problem for any view that would try to avoid the absolute beginning of time in the universe by postulating this primordial static state.

Kevin Harris: Yeah, and I can't figure out if he is theorizing that the history of the universe in fact is eternal and is infinite, and so on, because he seems to be indicating that there was a middle empty universe from which other universes sprang, and those universes which came out of this middle state have an arrow of time, but there's some kind of undifferentiated time as a coordinate or a component in this middle universe. And so there is in a sense a beginning or a center point there.

Dr. Craig: That's exactly right, Kevin, and it is reminiscent of Penrose's conformal cyclical cosmology, where time evaporates at the beginning, and so what you have are not other universes chronologically prior to ours, but simply different branches of a multiverse that have a common origin point, in which case you haven't avoided the beginning of the universe at all—you do have a beginning of the universe. And that middle state, as you say, cannot have existed for infinite time. As Carroll says here it's not completely stable, it has a half-life, it will decay, and he says, according to quantum mechanics things can fluctuate into existence, there's a probability of change occurring. And if there's any non-zero probability of a change occurring in a finite amount of time then it will have happened, given infinite time. So it seems to me that this cannot ultimately restore an eternal past, if that's the goal here.

Kevin Harris: He says something interesting. He says that he doesn't think that we can go to the past in time travel, but we go to the future all the time. He said, yesterday he went to the future and here I am; we're constantly going to the future and that's not a problem.

Dr. Craig: That's sort of, again, the arrow of time, that time has a directionality from earlier to later. But I think what this raises – and this was the reason I disagreed with him here on his analysis of what time's arrow is – I think that this is due to the reality of temporal becoming. You see, time is not simply structured according to earlier and later. I think it is also structured according to certain tenses, like past, present and future, and these are objective features of reality, not just observer-dependent notions. And it's not clear whether or not Carroll thinks that past, present and future are objective properties of events, as opposed to just subjective perspectival standpoints. He uses the word past and future a lot, but I suspect he's using that simply in a way to indicate earlier than and later than, than these actual tensed expressions which imply the reality of temporal becoming. And it seems to me that the anisotropy of time or the arrow of time is due to the impossibility of temporal becoming going backwards. Temporal becoming is always forwards in the sense that one event occurs after another, and that generates an arrow of time by the very nature of becoming.[3] So I see the arrow of time being rooted in a more fundamental property of time, namely the objectivity of temporal becoming, which gives rise to a structure of earlier than and later than. And it's not always clear exactly what Carroll is talking about when he mentions this primordial state. This primordial state would seem to have no entropy increase in it, but then he admits that that doesn't mean that there wouldn’t be any time. Time and space, he said, would still exist, and even says it would flow, even though there's no entropy increase and no one there to experience it. And I would agree with that, that if temporal becoming is an objective feature of reality then it's always going on whether or not anyone experiences the so called flow of time, and therefore this state that he postulates in which the universe is this sort of vacuum is a definitive temporal state characterized by temporal becoming, and therefore earlier than and later than relations.

Kevin Harris:Wired Magazine seems to kind of put words in his mouth a little bit by saying, “so there's an infinite number of universes behind us, and an infinite number of universes coming ahead of us.”

Dr. Craig: Yeah, I noticed that—he didn't say that.

Kevin Harris: No, he didn't say that.

Dr. Craig: That was putting words into his mouth—he never suggested that.

Kevin Harris: And then they say, “well, does that mean we can go forward and visit those universes ahead of us?” and stuff like that. And he answers that question, but doesn't really get to what they said before. He just said, “I suspect not; I don't know.”

Dr. Craig: Yeah, he thinks that because the arrow of time is from earlier to later, therefore you can't go back in time. But those who propound time travel in the context of relativity theory differentiate between the cosmic arrow of time of the whole universe, and the arrow of time in what's called 'proper time', which is the time of the individual observer. And it's true that your proper time never reverses and goes in the reverse direction, but nevertheless your proper time can always go forward, and in certain models of the universe that have been developed consistent with general relativity you would eventually come back to the point in space and time before you started and left. And so you would actually go to the past of cosmic time, even though you're always going forward in your own proper time. So it's not clear to me why we would reject that scenario. I don't think that's possible because I believe in the reality of temporal becoming. I think that temporal becoming is an objective world-wide feature of absolute time. But if you are a person who believes that the past, present and future are ultimately illusions of human consciousness, and that time is stretched out like a line, that it's more like a fourth dimension of space, then I think time travel becomes a much more plausible alternative, on that sort of view of time, the sort of static view of time, as opposed to a dynamic or tensed view. And I'm not sure which view of time he holds to.

Kevin Harris: Yeah, that's what I was going to ask.

Dr. Craig: There's nothing in the article that suggests that he holds that temporal becoming and tense are objective features of reality.

Kevin Harris: Sometimes known as A-theory or B-theory.

Dr. Craig: That's correct. Right, the A-theory would be the dynamic view on which there is real temporal becoming. The B-theory – which is the one that's prevalent among physicists – is the static model of time in which time is thought of as a fourth dimension of this geometrical entity called spacetime, and it's stretched out and just as real in its various parts as any spatial dimension is.

Kevin Harris: Film makers just don't like the A-theory of time – it's too boring for them – they want the B-theory so they can go back in time and go forward in the future.

Dr. Craig: Well, and they don't worry much about consistency, either, Kevin. Most of the time travel movies are just logically incoherent and make good entertainment, but they're not really consistent in the way they portray time travel. The best time travel flick I've seen is Bill and Ted's Excellent Adventure.

Kevin Harris: They were consistent. [laughter]

Dr. Craig: They were a consistent time travel movie, and that's what makes it such a delightful film.

Kevin Harris: He says, “to think of the multiverse not as a theory but as a prediction of a theory.”

Dr. Craig: Yes, that was an appropriate modesty. He says it's a prediction of my theory, but he says we're not even there yet. He says we don't know how to have a good theory, and we don't know how to test it. So we're not even to the point of saying these are predictions because we don't even have a good theory yet for this multiverse scenario, and we don't know how to test it even if we had it. So it all remains largely speculative.[4]